Homework Helpers: Physics

8 Waves and Light

One of the greatest debates in the history of physics concerned the nature of light. Was light made up of tiny particles, or did it travel in waves? The debate lasted for centuries, with scientists such as Sir Isaac Newton (1642–1727) and Albert Einstein (1879–1955) providing arguments for a particle theory, and Robert Hooke (1635–1703), Christian Huygens (1629–1695), and Thomas Young (1773–1829) favoring a wave theory. In this great debate, there was no clear winner. In the face of the evidence of numerous experiments, physicists have come to accept the wave-particle duality of light, meaning that light exhibits both wave-like and particle-like properties. In this chapter, we will take a closer look at the waves and also examine the wave-like properties of light.

Lesson 8–1: Types of Waves

To understand the wave-nature of light, we must discuss the properties and characteristics of waves first. We will start with a definition of waves, followed by a discussion of some of the types of waves.

A wave is defined as a disturbance that causes a transfer of energy over a distance, without a net transfer of mass. A type of wave with which you are probably familiar with is one that travels along the length of a taut rope when a person holding one end of it moves her hand up and down. Picture two people holding a length of rope between them. When one person holds his end steady and the other person moves her hand quickly up in a whip-like motion, a disturbance is created that will travel down the length of the rope until it reaches the other person. A single disturbance of this type is sometimes called a pulse. The person who was trying to hold his end steady will feel his arm jerk upward, as the disturbance reaches him. In this way, you can imagine the energy that has been transferred along the length of the rope. The fact that the far end of the rope grows no thicker, over time, shows that there is no net transfer of mass.

If one person repeatedly moves her end of the rope up and down, she produces what is called a continuous wave, periodic wave, or wave train, as a series of disturbances will travel down the length of the rope. If the motion of the person is uniform, you will see an alternating pattern of high crests and low troughs traveling along the length of the rope.

These waveforms aren’t limited to our example with ropes. If you keep your eyes open, you will see that these repeated patterns of disturbance occur in many materials, or media. To best understand a material medium, think about throwing a large rock into the water of a still pond. The rock strikes a certain spot, but the disturbance will spread out as ripples and affect a large area of the pond. A duck that was floating in the water several feet from where you threw the rock would eventually start to bob up and down, as the wave disturbance passed by. The water, in this example, acts as a material medium, allowing the disturbance to spread through it.

One way to classify waves is to distinguish between electromagnetic waves, which are waves that can travel through a vacuum, and mechanical waves, which are waves that require a material medium through which to travel.

If you put an alarm clock or some other source of sound under the bell jar of a vacuum pump, you could still hear the alarm, as long as there was still air in the bell jar. The alarm would set up vibrations or waves in the air molecules surrounding it. The air would act as a material medium, transferring the disturbance to the glass jar and vibrating its molecules. The molecules of the jar, acting as the new medium, would, in turn, transfer the disturbance to the molecules of air surrounding the jar. These air molecules would vibrate and pass the disturbance along until it reached your ears, disturbing the molecules of your eardrum.

What would happen if we turned the vacuum pump on and removed much of the air surrounding the clock, inside the bell jar? As we pumped the air molecules out of the jar, we would be removing the material medium that was passing the vibrations on to the molecules of the jar. We would notice the sound of the alarm diminishing, and it would eventually disappear altogether. The reason is because sound waves are mechanical waves, and they require a medium through which to travel. Despite what you may see in movies or television, sound waves can’t travel through outer space. If you were a spaceship pilot, and another spaceship flew quickly by your spaceship, you wouldn’t hear anything.

Could you see another spaceship from the window of your spaceship? Can light travel through outer space? Of course it can! How else would the sun be able to light our way during the day, or the moon and stars by night? Light has no trouble traveling through a vacuum, so it doesn’t fall under the definition of mechanical waves.

Early physicists attempting to defend the wave-theory of light “invented” an invisible material medium that surrounds the bodies of our universe, allowing them to argue that light was no different from other waves. They called this invisible medium that Earth floated in ether. The wave-theory of light was delivered a blow when an experiment designed to detect the ether failed to detect it. So a new category of waves that don’t require a medium to travel was suggested. Light is an example of an electromagnetic wave.

There are many other forms of electromagnetic waves, which, together with visible light, make up the electromagnetic spectrum. Radio waves, for example, allow communication between spaceships and Earth. When an astronaut sends a message to Earth, the sound waves that he or she produces are converted to radio waves and beamed to the planet, where they are then converted back into sound waves. Other examples of electromagnetic waves are given in the following table.

Another way to categorize waves is to distinguish the direction in which they travel in relation to the vibrating particles of the medium in which they travel through. Think about the duck we disturbed when we threw a rock in the pond a few paragraphs ago. The waves spread out horizontally, in the form of concentric “ripples” along the surface of the water, but the duck bobs up and down perpendicularly to the disturbance that passes it.

In transverse waves, such as the ripples in the pond, the wave travels in a direction that is perpendicular to the displacement of the particles of the medium. Light and all of the electromagnetic waves are other examples of a transverse wave.

Longitudinal waves travel along the same axis as the disturbance of the particles. When you compress an area of a spring, and then release it, the disturbance travels down the length of the spring, along the same axis in which the molecules of the spring are being vibrated back and forth. Areas where the molecules of a medium become more tightly packed are called compressions. The areas where the molecules become less densely packed than normal are called rarefactions. Sound is an example of a longitudinal wave.

Notice that these categories of waves are not all mutually exclusive. Light waves are both transverse and electromagnetic because they travel in a direction that is perpendicular to the disturbance, and they can travel without a material medium. Sound waves are both mechanical and longitudinal because they require a material medium through which to travel, and they travel in a direction that is parallel to the disturbance of the medium.

Lesson 8-1 Review

1. A _______________ is a wave in which the particles of the medium vibrate along back and forth along the same axis in which the wave travels.

2. A _______________ is a wave that requires a medium through which to travel.

3. A _______________ is an area in a material medium where the particles are less densely packed.